1. Field of the Invention
The present invention relates to a system and method for maintaining a constant infrared radiation output from an infrared radiation source despite variations in a resistance of the infrared radiation source, a voltage, or a current applied to the infrared radiation source.
2. Description of the Related Art
Gas analyzers are widely used in medical applications and are typically categorized into two different types: (1) “diverting” or “sidestream” gas sampling systems; and (2) a “non-diverting” or “mainstream” gas measurement systems. A mainstream gas measurement system includes a sample cell that is disposed along the main path of a breathing circuit through which a patient's respiratory gases flow and a gas sensing system coupled to the sample cell to measure the gas constituents. A sidestream type of gas measurement system transports a portion of sampled gases from the sampling site, which is typically a breathing circuit coupled to the patient's airway or directly at the patient's airway, through a sampling tube to the sample cell, where the constituents of the gas are measured by a gas sensing system coupled to the sample cell. A gas sensing system includes the elements necessary for monitoring respiratory gases including an infrared radiation (IR) source and detector. An example of a conventional mainstream gas measurement system is shown in U.S. Pat. No. 4,914,720 issued to Knodle et al. Examples of conventional sidestream gas sampling systems are taught in U.S. Pat. No. 4,692,621 to Passaro et al.; U.S. Pat. No. 4,177,381 to McClatchie; and U.S. Pat. No. 5,282,473 and U.S. Pat. No. 5,932,877 both to Braig et al.
Of considerable importance in gas sensing systems is the infrared radiation source (also referred to as an emitter), which produces the beam of infrared radiation. In non-dispersive gas analyzers, such as disclosed in the '720 patent, the emitter has resistive ink bonded to a substrate of a material with a low thermal conductivity that is electrically-pulsed in unipolar or bipolar fashion. High intensity infrared energy is emitted from the resistive ink portion of the emitter and is either reflected by a mirror and then collimated or directly collimated by a lens. With conventional infrared sources, the applied emitter voltage (Ve) must be tightly regulated by voltage regulators to achieve a consistent source output. Voltage regulators known in the art are either of the linear or switched mode type. Linear mode voltage regulators may consume greater than 30% of the power provided to them, leaving only at most 70% of the power available for to drive the emitter. While switched mode voltage regulators may consume only 10-15% of the power provided to them, they are complex and expensive.
To further complicate matters, the resistance of the emitter is not constant in each emitter in a batch of emitters due to variances in the manufacturing processes. Manufacturers may accommodate this variation in the output of the emitter by using a larger input dynamic range for the signal receiving front end of the detector portion of the gas sensing system. Additionally, the resistances of such an emitter may slowly change over time, and, therefore, so does the infrared radiation output of that emitter. The change in the resistance of the emitter can be measured by monitoring changes in the current flowing through the emitter (Ie). As a result of these variations, if either the emitter voltage or the emitter current differed from factory calibration conditions, it was possible for the output of the gas analyzer to yield results outside of its stated specification.
Given these problems with existing infrared radiation sources, it is desirable to provide an IR source that can adapt to differences in the input driving voltage and differences in the properties of the infrared radiation source, including but not limited to changes in the IR source resistance, and, at the same time, permit the infrared radiation intensity and spectral power distribution of the infrared radiation output by the IR source to remain substantially constant. It is also desirable to provide all of the advantages of such an adaptive IR source without the need to utilize a voltage or current regulator that adds cost and complexity, and wastes power.